The outer membrane of gram-negative bacteria is comprised of a mosaic of tightly associated lipopolysaccharide (LPS) and proteins. In some organisms, the outer membrane is covered by loosely bound polymers, the bacterial capsule. Since it is the surface of the bacterium which interacts with the host in early stages of parasitism and since it is host defenses directed towards surface constituents which play a role in the prevention and/or resolution of bacterial infection, we are engaged in comparative studies of the structure and function of these outer membrane constituents in three venereal pathogens-Neisseria gonorrhoeae, Chlamydia trachomatis and Contagious Equine Metritis Organism (CEMO). Studies of N. gonorrhoeae include those of LPS, LPS/protein I complexes, H8 antigen (a surface-exposed lipid-protein complex), and capsule (a large molecular weight carbohydrate polymer); studies of C. trachomatis include those of the LPS; whereas the LPS and capsule of CEMO are being investigated. Gonococcal LPS lacks O side chains but is remarkably heterogeneous both structurally antigenically vis-a-vis the core oligosaccharide. Gonococcal LPS is closely associated with the major outer membrane porin protein (P.I.). This association is impervious to anionic detergents at high temperatures. Immunization with P.I. antigen elicits antibodies to P.I. and LPS. The gonococcal H8 antigen is an abundant, immunogenic surface-exposed antigen common to all strains of pathogenic neisseria. Although the existence of a gonococcal capsule has been the subject of great debate for decades, we have recently obtained morphological evidence (SEM and TEM) of such a capsule. Concurrently carbohydrate polymer has been isolated from cultures of gonococci and is presently being analyzed. The rough LPS of C. trachomatis has a lipid A backbone similar to that of enteric lipid A. Unique long-chain fatty acids, phosphorous, KDO and glucose are also present in this nontoxic LPS. The LPS of CEMO is also rough; some colonial morphotypes produce a capsule which is antigenic and immunogenic in the experimentally infected horse.